In the present research, we have fownd that individual single duplex DNA chains undergoes large discrete phase transition between elongated coil and compacted globule (coil-globule transition), based on the single chain observation by use of fluorescence microscopy.Whereas ensemble of DNA has the characteristics of diffuse or continuous transition. From single chain observation, we found that the first order phase transition is induced by various kinds of condensation agents. As for as we know, this is the first finding on "nucleation & growth" from a single polymer, not only DNA but also all the other synthetic and natural polymers. In addition to this, the relationship between gene-transfection efficency and higher-order structure of the DNA carrier-complex was discussed.